Technical Field
The instant disclosure relates to a transformer circuit for applying to a radio-frequency power amplifier in a communication system, and more particular to a transformer circuit with wide frequency-bandwidth and low current-resistance drop.
Related Art
With the blooming developments of IT industries, the speed for data transmission is required to be faster, thus the long-term evolution (LTE) of 4G techniques becomes popular. However, because the number of the frequency-bands supported by the LTE technique are much more than those supported by the conventional 2G technique, the telecommunication device has to be improved to support multiple operating frequency bands.
Conventionally, a number of single frequency-band power amplifiers are utilized in the telecommunication device for supporting different operating frequency bands. For long, the conversion efficiency of the transformer circuit is deemed as the focus of the specification of the power amplifier. The main factor resulting in the efficiency reduction of power amplifier modules is the current-resistance drop (IR-drop) of the transformer circuit itself. Furthermore, along with the evolutions of the complementary metal oxide semiconductor (CMOS) manufacturing techniques, the size of the transistor is reduced continuously while the price of the chip per unit area is increased significantly. In addition, the power combiner of the power amplifier usually utilizes magnetically coupled passive components which do little effort on the size reduction in advanced CMOS manufacturing techniques (such as 90-nm manufacturing techniques) but have higher price; therefore, the area of the passive components or the transformer for power combining must be small for cost reduction.